This invention relates to a semipermeable membrane support of the type which is used for precise filtrations such as in ultra-filtration and reverse osmosis. The semipermeable membrane comprises a non-woven fabric.
The demand for precise filtration is continually increasing. Such precise filtration is widely utilized in the fields of desalination of sea water, the food industry, treatment of industrial waste water, and in medical applications.
Semipermeable membranes comprising synthetic polymer, without any support, are inferior in mechanical strength. Therefore such membranes are generally used with a backing material such as woven fabric. Recently, the membrane has been formed by casting a polymer solution directly on a porous support such as a non-woven fabric.
The conventional supports generally experience a peeling off problem due to inferior affinity between the membrane and support. Another problem of conventional supports is that of pin-holes resulting from insufficient penetration of polymer solution into the support. The conventional supports also display a lack of uniformity in mechanical properties such as strength.
Various modifications have been proposed to eliminate these defects. One such method described in Japanese Patent Publication No. 52 (1977)-15398, involves dissolving a part of the support with a polymer solution. Another approach, as described in Japanese Patent Provisional Publication No. 55 (1980)-132065, is in increasing the affinity by previous immersion of the support in the same solvent as used for the polymer solution. Japanese Patent Provisional Publication No. 58 (1983)-49408 suggests stepwise casting of a high viscous solution and a low viscous solution.
Conventional supports are generally of woven and knitted fabrics, non-woven fabric, porous sintered material, or paper. However, no suitable base material especially for semipermeable membrane is available. Even and dense base materials for other uses are selected as semipermeable membrane support materials.
Thus, the prior art lacks a support specifically suitable for forming semipermeable membranes. Conventional modifications require unusual processes or methods to form the membrane. It has been found that when one attempts to increase the affinity, the polymer solution is apt to penetrate through the support to the back surface and interfere with the forming of the membrane.
The method wherein a part of the support is dissolved in a polymer solution to increase the affinity requires the selection of soluble material in the support for each solvent of the polymer solution. This is not always utilized in a wide range of polymer solution for forming semipermeable membranes, and does not have a broad spectrum of usefulness. Additionally, improving the affinity by dissolution requires some period of time after the casting of a solution. Therefore the coagulation process cannot be carried out rapidly.
The above-mentioned method of Japanese Patent Provisional Publication No. 58 (1983)-79506 in which desized fabric is used involves essentially the combination of a polymer solution. This method is not always suitable to a wide range of polymer solutions for forming semipermeable membranes and thus also lacks a broad spectrum of usefulness.
In addition to the above-discussed methods, various supports which use non-woven fabric have been developed. However, supports having a high density cause insufficient penetration of the polymer solution. A consequence is the delamination between layers due to reduced peeling strength between the support and membrane. Another consequence can be the generation of pin-holes due to residual bubbles, because of insufficient de-bubbling in the support. Low density supports, in spite of sufficient penetration of a polymer solution, result in over penetration of the polymer solution to the back surface opposite to the casted surface. The resulting membranes give uneven filtration. Severe defects such as reduced filtration performance and/or damage of the semipermeable membrane due to the partial excessive pressurization during filtration operation can result.
Support materials such as paper are inherently high in density and therefore have inferior penetrability. Modified paper material of reduced density, to improve the penetrability, may cause fuzz and consequently pin-holes. Thus, a uniform membrane cannot be formed using paper material.